Cathode-ray tube



May 20, 1958 DE 15R ET AL 2,835,838

CATHODE-RAY TUBE Filed July 12, 1954 INVENTORS Johumes De G|er I l-fmJ Franz Konr d Johann flit/d5 Alphas: Petr; Van Ro y k/ll/ln Fat/:0 N/enhws 2,835,838 CATHODE-RAY TUBE Johannes De Gier, Eindhoven, Netherlands, Wilfrid Franz Konrad Johann Niklas, Milwaukee, Win, and Alphonsus Petr-us Van Rooy and Willem Foklro Nienhuis,'Eindhoven, Netherlands, assignors, by mesnc assignments, to North American Philips Company, Inc., New York, N. Y., a corporation of Delaware Application July 12, 1954, Serial No. 442,724 Claims priority, application Netherlands July 18, 1953 4 Jlaims. (Cl. 313-82) This invention relates to devices comprising cathoderay tubes, for, example for television or oscillography purposes, and also to cathode-ray tubes intended for use in such devices.

In cathode-ray tubes intended, for example, for the above-mentioned purposes, the electron current emitted by a cathode is focused or concentrated with the use of a magnetic or electrostatic electrode system. Furthermore, a combination of magnetic and electro-static focusing is frequently found, more particularly in modern tubes of this kind.

It is known that the beaming may advantageously be effected in two stages, which will be referred to hereinafter as main concentration and pm-concentration. The invention solely relates to devices in which the preconcentration is effected electro-statically; the main concentration may take place either electro-statically or magnetically.

Different electrode systems may be used for pre concentration. The two kinds commonly used are the so-called triode system and the tetrode system. In the first-mentioned system, the cathode and the collector electrode, for example the luminescent screen, have arranged between them primarily an electrode having a small aperture, which electrode is in most cases referred to as the grid, and secondly an electrode having a high positive voltage with respect to the cathode and thus determining for at least 50% the positive voltage gradient at the cathode surface. The field of the last-mentioned electrode, which is in most cases referred to as the anode, for this purpose penetrates throughthe aperture of the grid to the cathode surface. In a tetrode, another electrode, in most cases referred to as the accelerating anode, is provided between the ,anode and the grid, and in this system the positive voltage gradient at the cathode is determined for more than 50% by the potential of the accelerating anode.

In both the triode system and the tetrode system, the grid is given a negative bias with respect to the cathode. Furthermore, alternating voltages are frequently applied to this grid, in order to vary the intensity of the electron beam.

In a tetrode system a strong positive lens is formed between the accelerating anode and the anode. This lens has the disadvantage of involving strong so-called modulation defocusing. This may be explained as follows.

Close to the aperture of the grid a contraction of the beam occurs, which will be referred to here as crossover in accordance with the term used therefore in the English and the American literature. This cross-over serves as an object in the reproduction. It is to be noted here that the cross-over is not a mathematical point, but has dimensions, although small, at right angles to the direction of propagation of the electrons in the beam. These dimensions and also the distance between the cross-over and the grid are dependent inter alia upon the potential of the grid. The potential of the grid and hence the position and size of the cross-over vary as a 2,835,838 Patented May 20, 1958 result of the control-voltages applied between the cathode and the grid. It will be evident that this influences the projection brought about by the main concentration system. This projection, once adjusted, provides a determined size of the cross-over on the collector electrode, for example the luminescent screen, and this size thus varies due to the control-voltages applied to the grid. This is termed modulation defocusing and is an undesirable phenomenon. In the tetrode system the lens between the accelerating anode and the anode is active as a magnifying glass, the variation in position and size of the cross-over thus being projected by the main concentration system on an enlarged scale. It will be evident that the said modulation defocusing is less troublesome in a triode system, since a lens which is active as a magnifying glass is not present before the main concentration system. However, the triode system is less satisfactory than the tetrode system in one particular respect, as will be explained hereinafter.

As mentioned above, the cross-over is projected on the screen by the main concentration system. Now, it is a known fact that the smaller the cross-section of the electron beam at the main concentration system, the smaller is the so-called deflection defocussing. The term deflection defocusing is to be understood to mean that the size of the cross-over on the collector electrode, for example theluminescent screen, increases for large deflection angles of the beam. This is naturally an undesirable phenomenon. The tetrode system provides a beam having a cross-section at the main concentration system smaller than that of a beam produced by a triode system. Consequently, as mentioned earlier, the triode system provides smaller modulation defocusing, and, as just shown, the tetrode system provides smaller deflection defocusing.

The invention relates to a device comprising a cathoderay tube in which the are-concentration system is constructed in a manner and fed by voltages such that the whole of the concentration system exhibits the small modulation defocusing of the triode system and the small deflection defo'cusing of the tetrode system.

A device according to the invention comprises a cathode-ray tube in which the electron current emitted by a cathode is focused by a main concentration system and a preconcentration system, which is provided between the cathode and the main concentration system and which, together with the cathode, constitutes a triode system in which the distance between the grid surface and the aperture in the anode on the side of the cathode is at least equal to 1.5 times the diameter of this aperture. The invention is characterized in that the space between the grid and the anode of the triode system is substantially wholly surrounded by an additional electrode which has applied to it a biasing potential at least equal to that biasing potential of the grid which suffices for complete suppression or cut-off of the electron current and which is at the most equal to the opposite of twice this potential.

In a tube for use in a device according to the invention, there are consequently provided three electrodes between the cathode and the main concentration system,-

but nevertheless there is no question of a tetrode system existing, since the requirement must be fulfilled that the positive voltage gradient at the surface of the cathode is determined for more than 50% by the anode and not, as in a tetrode system, by the second electrode, as reckoned from the cathode.

If the biasing potential applied to the additional electrode would be chosen unduly high, that is to say higher than the above-mentioned limit, a gradual transition to a tetrode system would result, whereby the potential of the additional electrode would determine for more than 50% the positive gradient at the cathode surface. The additional electrode is even preferably given a negative biasing potential with respect to the cathode, which potential is at the most equal to the bias of the grid. For normal tubes, the grid voltage varies between and 50 volts negative with resepct to the cathode; the cutoff voltage, that is the voltage of the grid at which there is no flow of electrons, is comprised between -30 and 80 volts according to the construction of the tube. Consequently, the biasing potential of the additional electrode may vary between +160 volts and --80 volts.

By fulfillment of the above-mentioned requirements, it is achieved that the electrode system provides a modulation defocusing smaller than that of the tetrode system, While nevertheless.the beam produced is so narrow that its cross-section at the main concentration system is so small that the deflection defocusing is considerably smaller than that of the same system without the additional electrode and hence of a normal triode system.

The three electrodes of the pre-concentration system in a tube according to the invention may have difierent forms. As a rule, the grid comprises a plate having a small aperture, the additional electrode and the anode in most cases being co-axial cylinders of difierent or equal diameters. In order to ensure that the field produced by the anode penetrates as far as the surface of the cathode, the aperture of the additional electrode on the side of the grid is preferably made considerably larger than the aperture of the grid. In the tetrode systern the apertures in the grid and in the accelerating anode on the side of the grid are in most cases of the same order of magnitude, the aperture in the accelerating anode in most cases having a diameter comprised between the diameter or twice the diameter of the aperture of the grid. In the additional electrode of the system in a tube according to the invention, the aperture on the side of the cathode is preferably tive to thirty times the size of the aperture in the grid.

In a device according to the invention, the main concentration system may operate either electro-statically ,or magnetically.

In order that the invention may be readily carried into efiect, it will now be described with reference to the accompanying drawing in which a cathode-ray tube according to the invention for reproducing television pictures is shown diagrammatically and not to scale.

In an envelope 1 a transparent window 2 is covered with a luminescent screen on which the picture may be reproduced with the aid of a focused electron beam emanating from a cathode 3.

Between the luminescent screen and the cathode 3 there is arranged the are-concentration system comprising electrodes 4, 5 and 6, and the main concentration system comprising a focusing coil 7 which externally surrounds the tube. The electron beam may be deflected by deflection coils 8 and 9. The electrode 4 is the grid which has a small central aperture 10. This electrode is given a biasing potential which is negative with respect to the cathode 3. The anode of the PIC-COHCBIItIZltiOII system is constituted by the cylindrical electrode 6 which has a high positive bias e. g. 1-10 kv. applied to it. The additional electrode arranged between the grid and the anode is indicated by 5 and has applied to it a biasing potential at least equal to that bias of the grid 4 which sufiices for complete suppression or cut-off of the electron current and which is at the most equal to the opposite of twice this potential. As may be seen from the figure, the electrode 5 surrounds the space between the grid 4 and the anode 6 almost completely and also surrounds the end of the latter. As a matter of fact, there must be sufiicient space on the one hand between the grid 4 and the electrode 5 and, on the other hand, between the anode 6 and the electrode 5 to avoid a short-circuit and arcing. The distance between the aper- '4 ture of the anodeon the side facing the grid and the surface of the grid is indicated by 11; it is at least equal to 1.5 times the diameter of the aperture in the anode on the side of the grid, which isindicated by 15.

In order to illustrate the ratios of the various biasing potentials, we have indicated schematically a circuit for operating the tube. Therein 12 is a source of high potential, e. g. 11() kv. which is shunted by a potentiometer 13. The cathode 3 is connected to a tap A thereon and the bias for the grid 4 may be regulated by a variable tap B on the negative side of the potentiometer 13. A second potentiometer 14 is connected across the part between taps C and D of the potentiom eter 13, which points are so chosen that C is the cut-off grid bias value of the tube and D provides a positive bias which is twice the negative value of point C. By means of a variable tap E on the potentiometer 14 the bias of electrode 5 may be varied between the negative bias of point C and the positive bias of point D. As shown, the anode 6 is connected to the positive or highpotential end of the source 12.

What is claimed is:

1. In a cathode-ray tube, a pre-concentrating triode electrode system comprising a cathode, an apertured control grid, and an apertured anode arranged in that order and aligned with one another, the aperture in the anode facing the cathode being spaced from the grid surface a distance at least equal to 1.5 times the diameter of the aperture in the anode, an additional electrode having a diameter exceeding that of the anode and substantially surrounding the space between the grid and anode, means for supplying potentials to said cathode, grid and anode, said potential applied to said anode having a value at which at least half of the resultant field gradient at the cathode surface is determined by said anode, and means for supplying a potential to said additional electrode in the range between a negative potential which when applied to the grid would cut-ofi the tube and a positive potential equal to twice the cut-0E potential.

2. A cathode-ray tube as set forth in claim 1 wherein the potential applied to the additional electrode is negative relative to that applied to the cathode but is not more negative than the potential applied to the grid.

3. A cathode-ray tube as set forth in claim 1 wherein the aperture of the additional electrode on the side adjacent the cathode has a cross-section in the range between five and thirty times the cross-section of the grid aperture.

4. In a cathode-ray tube, an electron gun for producing a focused beam of electrons, said gun comprising a cathode, an apertured control grid, a tubular anode having an aperture of given size, and a tubular additional electrode having a diameter exceeding that of the tubular anode and substantially surrounding the space. between the control grid and anode and including a portion surrounding the adjacent end of the anode, and means for biasing the gun electrodes such that the anode is maintained at a high positive potential relative to the cathode. the control. grid is maintained at a low negative potential relative to the cahode, and the additional electrode is maintained at a potential not exceeding cathode potential.

References Cited in the file of this patent UNITED STATES PATENTS 2,152,825 Schlesinger Apr. 14, 1939 2,209,159 Gorlick et a1. July 23, 1940 2,223,040 Mahl Nov. 26, 1940 2,225,901 Bruche Dec. 24, 1940 2,233,299 Schlesinger Feb. 25, 1941 2,306,663 Schlesinger Dec. 29, 1942 2,389,903 Hahn Nov. 27, 1945 2,476,060 Moss July 12, 1949 2,490,308 Klemperer Dec. 6, 1949 

